1. Field of the Invention
This invention relates generally to mechanical end face seals and more specifically to such seals used in devices for containing process fluids under high pressure, and especially for containing process fluids including highly viscous liquids, the devices accommodating for distortion caused by thermal gradients and/or by high pressure differentials between the outer and inner diameters of such seals.
2. Background Art
Mechanical end face seals have become high technology items utilizable in a variety of industries. These types of seals are designed with a great deal of care and attention to the materials, shapes, dimensions and tolerances of the component parts. Such attention to detail in the design is necessary in order to accommodate a great number of characteristics, any of which characteristics is liable to affect the efficiency or operation of seals of this type. Minor changes or alterations to any one of about 10 physical features of a seal, its components or its sealing environment may, and in most cases will, result in a change in the sealing characteristics, sealing capability, wear, endurance and/or material integrity.
In most cases, it is desirable for the seals of this type to operate maintenance and trouble free for extensive periods of time, on the order of years. The seals are used in machinery which cannot be shut down for long without severely impairing the operation and efficiency of, for example, a large chemical plant or refinery.
For contacting seals, which seal devices contain highly viscous liquids, such as oils or petroleum, a different set of considerations occurs because of the high levels of shear heating which develop between the seal faces. In applications which are required to handle two or more types of liquids at different times, the fluids having different fluid properties, at varying pressures and other sealing conditions, may result in uneven or counterproductive seal operation. For example, highly viscous liquids, such as crude oil, produce frictional or shear heating at the seal faces, which translates into a temperature and/or pressure phenomenon in the seal ring known as coning.
It has been recognized that uneven heating may result in thermal distortion of a seal ring and that high pressure differentials between the inboard and outboard diameters of the seal rings may result in pressure distortions of the ring material. Such distortions are undesirable because they cause the normally flat opposed mating seal faces of the seal rings to diverge from the normal sealing engagement between the seal ring faces. In a non-contacting seal, distortion of the seal rings normally causes the outer diameters of each seal ring face to rotate about the centroid toward the centerline of the seal ring, causing the seal gap to become wider at the inner diameter of the seal ring interface and to narrow at the outer diameter. This seal ring distortion and change in axial depth of the seal ring gap is not conducive to optimal sealing capability, because the effects of the spiral grooves pumping a gas against a dam are dissipated if the seal gap is not minimized and the dam is not adjacent the opposed mating sealing face. The narrowing of the gap at the outer diameter is undesirable for a non-contacting seal because a smaller gap renders the seal faces susceptible to undesirable contact, resulting in premature wear of the seal faces.
Coning is an undesirable phenomenon which occurs from heat differentials or gradients occurring between axial ends of a seal ring. Excess generated heat causes the seal ring material to expand at different rates depending on temperature, resulting in material expansion at different rates and in seal ring distortion. Typically, for a conventional seal, the distortion results in the seal faces at the interface separating at the outer diameter. As the seal ring material expands faster at the seal face end, the annular seal rings take a slightly conical shape; hence, the designation of this phenomenon as coning.
Convex distortion, or coning, develops when fluids are sealed that are viscous, such as oil. Such fluids are subjected to shear heating in the gap between the opposed seal faces of the seal rings. Due to the seal characteristics, uneven temperature distribution develops over the seal rings, and the interfacial gap between the two seal faces becomes larger at the outer diameter than at the inner diameter, as a result of differences in the expansion of the seal ring material.
Several disadvantages result from the uneven ring distortion and from the resulting gap difference in the seal gap between the inner and outer diameters. For seals which are pressurized at the outer diameter, the increased gap permits fluid, which is at a high pressure, to enter the seal interface and thereby to increase the hydrostatic opening force. This leads to a greater film thickness and a higher than desirable leakage rate.
Conventional approaches addressing the coning problem have included increasing the closing force, either by increased spring load or providing a higher balance ratio, or by relying on a concave pressure distortion of seal rings which are pressurized at the outer diameter. Increasing the load so as to urge the seal rings more forcefully toward each other minimizes the interfacial gap, but results in even more shearing heat generation, which leads to even greater coning problems. The closing force which is required to reduce the film thickness, and consequently leakage, must increase with fluid viscosity. Eventually, further increases in the load results in high seal face temperature, which collapses the oil film and may cause unwanted rubbing contact of the seal faces. At this highly loaded condition, the seal ring materials are likely to fail from severe wear, carbon blistering or carbide heat checking.
Alternatively, it has been found that concave distortion of outer diameter pressurized seals can be modified by changing seal cross-sectional geometry. One example of such a pressure-induced concave distortion, calculated to counter and compensate for thermal distortion, has been proposed by Lebeck et al. and is the subject of U.S. Pat. No. 4,792,146. A "thermal-net taper" is disclosed by Lebeck et al. and relies on a specified geometry which is claimed to match the distortion of each of the rings so that the seal faces remain parallel under the predetermined sealing conditions. The seal faces are described as remaining in a parallel relationship over a large range of heat distribution and pressure distortion parameters. As described, predetermined parameters for the materials, and other factors, limit the number and range of applications available for use of seals having the Lebeck et al. characteristics and structures.
Similarly, U.S. Pat. No. 5,135,235, issued to Parmar and assigned to a company related to the assignee of the present invention, also utilizes a distinct cross-sectional configuration calculated to cause desirable distortion of the seal faces so as to maintain seal face parallelity over a range of localized temperatures.
Although the types of arrangements discussed in U.S. Pat. Nos. 4,792,146 and 5,135,235 are useful for applications in which the sealing conditions are mostly predictable, e.g., applications including constant pressure differentials across the seal faces, when conditions are variable or sudden changes in the sealing conditions raise additional considerations which render these configurations less than optimal. For example, in conditions where the pressure is low, or the shaft rotational speed is higher than optimal, the heat generated by shear heating and frictional contact is excessive to the detriment of the seal's capability to maintain the seal faces parallel to each other.
Conflicting requirements in seal design are encountered for seals utilized at extremely high pressures, i.e., in excess of 1800 psi. The pressure forces acting on such seals necessitate a large thickness of the seal ring in order to provide rigidity to withstand the high pressures. Likewise, the rigidity of a thick ring detracts from flexibility and cannot easily provide a compliant seal face. The thicker rings distort in accordance with the teachings of the above described patents. If such a seal is also expected to operate at low pressures as well, an optimal configuration for maintaining the seal faces parallel becomes extremely hard to achieve, since typical pressure distortions at low pressures are insufficient to overcome expected distortions resulting from thermal heating in known seal ring configurations.
In the context of a non-contacting type seal, commonly assigned U.S. Pat. No. 3,804,424 describes a gas seal having thermal and pressure distortion compensation. The seal relies upon a number of orifices passing through one of the rings to provide pressurized fluid to the seal interface. The orifices are in communication between the high pressure fluid being sealed at the outer diameter of the seal and a chamber between an inner and outer diameter dam of the primary ring. The resulting increase in pressure in the seal interface provides for an even pressure differential across the interface.
Minimization of seal face distortion has been addressed often, most recently in related and commonly assigned U.S. Pat. No. 5,681,047, which is utilized in non-contacting seal applications. U.S. Pat. No. 5,681,047 illustrates and describes a non-contacting type seal for sealing relatively inert gas, in a groove-type seal. The problem addressed by the patent relates to high-pressure differentials across the seal faces and the distortion caused by that differential.
Other attempts at correcting for seal face distortion are disclosed by U.S. Pat. No. 5,755,817, drawn to a pump configuration having a hydrostatic seal which includes a seal element with a recess in the seal body of from approximately 0.10 inch and about 0.15 inch depth. The location of the recess along the seal element can be altered to alter the control over flexing, but caution is taught in order not to create excessive stress concentrations within the seal element. The recess is considered to resist tapering effects and to provide a seal that is responsive to both temperature and pressure effects.
Accordingly, what has been found necessary for seals that undergo variable conditions, or for seals which are intended to seal highly viscous fluids, is a highly compliant seal face configuration, in conjunction with a rigid seal ring construction capable of withstanding higher pressures, if necessary. The compliant seal face configuration ideally compensates for a wide range of pressure and temperature conditions and can be used in any of a number of seal configurations.